Rotary mills are well known for milling grains and other products. In general, these mills introduce a coarse product into a milling chamber whereupon it is ground to a fine particle size between a hammer and a screen. After grinding, fine particles pass through the screen and the milled product is removed from the milling chamber.
In the past, while such rotary hammer mills are effective in producing milled product, they are limited in a number of ways with respect to the efficiency of milling. For example, a hammer mill grinds the product against the screen in order to pulverize the product to a sufficiently fine size that the product can pass through the screen. While this is effective in producing a uniform product, it is destructive to the screen requiring frequent replacement of the screen due to wear. In order to address this problem, past screens systems have been developed wherein the product, upon entry into the mill, is initially abraded against a solid surface called a wear strip prior to finer grinding against the fine screen.
Furthermore, the efficiency with which such a mill operates is dependent predominantly upon the passive diffusion of a small particle through the screen. This results in limited volumes being processed as a result of both ineffective passage of product through the screen by non-active means as well as increased temperature of the product. As a result of the passive diffusion of particles, there is a tendency for the temperature within the mill to rise, leading to potential product breakdown. In the case of grains, this may lead to a loss in nutritional value as well as to an increase in humidity within the mill which can cause gumming problems. Overheating may also cause the temperature in the milling room to increase uncomfortably.
Other limitations of past hammer mills include the difficulty and expense of replacing worn screen assemblies. Typically, the fine screen of a screen assembly having both a coarse and fine screen will experience faster wear requiring regular replacement of the fine screen. Past screen assemblies have been limited in the ease of replacement of the fine screen often requiring complete disassembly of the screen assembly to remove and replace a worn fine screen. Some screen assemblies waste screen material through unnecessary overlap. Furthermore, in the past, the wear strip has been attached to the mill which complicates its replacement particularly if it has been welded to the mill.
Still further, in a milling operation, small animals such as rodents such as mice may be attracted to the milling room and rotary mills because of the inevitable spillage of product. During shut-down of the milling room operations, rodents may attempt to enter a milling machine in search of food which, if undetected, will severely compromise the quality of product upon start-up.
In another but related area, after becoming worn, rubber vehicle tires are discarded, usually in designated tire dumps. As is well known, these tire dumps are ecologically damaging and wasteful. The ability to recycle these tires is limited in view of the multi-component structure of a tire and the difficulties in effectively utilizing the tire after use. The typical tire has a steel belted tread with rubber/nylon sidewalls. This construction presents significant difficulties for the potential re-cycling of the rubber in the tire.
Accordingly, there has been a need for rotary mills which address the above problems, namely improving product processing rates, minimizing screen wear, reducing the time required to remove screen assemblies from a mill and replacing the fine screen, which do not overheat the product and which do not result in gumming or clogging the screen assembly during operation.
Still further there has been a need for rotary mills which minimize the risk of animal or rodent entry and which may be used to effectively mill other products such as rubber from discarded tires.
In addressing these problems and, in particular, improving processing volumes, there has been a need for a mill having cutting knives designed to cut product as opposed to hammer or crush the product against a screen. In order to further promote higher processing volumes, there has been also been a need for a mill having cooling fins to promote cooling, product removal and an air-inflow system which prohibits rodent entry. Still farther, there has been a need for a mill designed for simplified maintenance wherein the screen assembly including a wear strip can be quickly and easily rotated within the mill or replaced without wasting screen material. There has also been a need for a mill which can be easily adapted for milling rubber.
Examples of previous devices and assemblies include Canadian patent application 2,091,954 in which a screen construction for a mill is described wherein a fine screen is supported by a coarse screen in order to reduce the flexibility of the screen. However, this machine is limited in the volume and types of material or product which can be processed. It is also has problems with respect to excessive heating of the product during grinding leading to a loss in nutritional value of the product.
Canadian patent 1,222,234 describes a mill producing a fine flour which addresses the dusting and dough problems of previous mills around the air intake in the door. The design of this air intake, however, is overly complex and still results in some spillage of product.